From: David Alan Black <dblack@...>
Date: 2004-05-12T09:53:51+09:00
Subject: Re: Strange regexp behaviour in gsub

Hi --

Kristof Bastiaensen <kristof@vleeuwen.org> writes:

> On Wed, 12 May 2004 08:15:28 +0900, Joel VanderWerf wrote:
> 
> > Kristof Bastiaensen wrote:
> >> Hi,
> >> 
> >> I am wondering if this is the correct behaviour in gsub:
> >> 
> >> "bab".gsub(/(?!a)ab/, "cd")
> >> => "bab"
> >> 
> >> shouldn't that be "bcd"?
> > 
> > I think /(?!a)ab/ can't match anything. It's saying that the first 
> > character after the beginning of the match must not be "a", and the 
> > first character of the match must be "a". This is contradictory.
> 
> Yes, that would clarify the situation, but is it the correct
> behaviour?  I would think that (?!a)a doesn't mean the same
> character, but consecutive ones.  Because it doesn't consume the
> character, it effectively is the character 'before' the match (if
> any).  

/(?!a)/ doesn't match or consume any character; it refers to the state
of things between characters.  The previous character (or start of
string) has come and gone; the assertion, now, is "what lies just
ahead is not 'a'".  

> The other behaviour wouldn't make sense, because (?!a)b is then
> exactly the same as b.

Assertions like this always have the possibility of being redundant --
for example:

  /(?=a)abc/  # same as /abc/

but there are a lot of cases where they aren't, and that's where they
become useful:

  /David (?!Black)(\S+)/  # grab another David's last name


David

-- 
David A. Black
dblack@wobblini.net